1. Field of the Invention
The present invention relates to a process for preparing a polyurethane foam. More specifically, the present invention relates to a process for preparing a polyurethane foam which can be suitably used as shoe soles and the like.
2. Discussion of the Related Art
Polyether-based polyurethanes have been employed in soles of men's shoes which would be used for a long period of time since the polyether-based polyurethanes are excellent in hydrolysis resistance as compared with polyester-based polyurethanes. However, the polyether-based polyurethanes are poor in mechanical characteristics such as abrasion resistance and flexing properties at ambient temperature.
To improve the mechanical characteristics of the polyether-based polyurethanes, it has been proposed to replace all or a part of polyoxypropylene-based polyols with polyoxytetramethylene glycol as a polyol component. In general, however, it is difficult to apply a sole made of a polyether-based polyurethane, particularly a sole having a low density to practical uses since the strength of the soles is generally insufficient not only in its foaming process but also as a final product.
Japanese Patent Laid-Open No. Sho 61-268716 discloses a process for preparing an inflexible polyurethane foam excellent in air-permeation properties and dimensional stability, wherein a low-molecular weight polyol and a high molecular weight polyol are used in combination, and not less than 1 part by weight of urea and not less than 2 parts by weight of water are simultaneously used based on 100 parts by weight of the polyols. However, this document does not disclose or suggest the use of a specific amount of urea to improve the strength of a polyurethane foam.
Japanese Unexamined Patent Publication No. Hei 4-502926 discloses that an alkoxylated polyol containing urea in a content of about 5 to about 15% by weight is used in the production of an improved flame-retardant polyurethane foam, wherein the alkoxylated polyol is prepared by subjecting an urea-dissolved mixture obtained by dissolving urea in the mixture of a polyether-polyol or a polyester-polyol with a sugar such as sucrose or lactose to alkoxylation using an alkylene oxide. However, in the alkoxylated polyol, a relatively large amount of urea is used. Also, this document does not disclose or suggest the use of urea in a specific amount to improve the strength of a polyurethane foam.
An object of the present invention is to provide a process for preparing a polyurethane foam having a high strength in spite of having a low density, and is excellent in demolding properties.
Another object of the present invention is to provide a process for preparing a polyurethane foam which can be suitably used as shoe soles.
These and other objects of the present invention will be apparent from the following description.